TRACE_EVENT_PHASE_COMPLETE (X Events)

An X event combines a pair of the original B and E events.
It can reduce the trace size by about half.
Changed TRACE_EVENTx() macros to generate X events.

BUG=trace-viewer:142
TEST=Existing TraceEventTextFixture.* tests
TEST=TraceEventTextFixture.TraceBufferRingBuffer
TEST=TraceEventAnalyzerTest.CompleteDuration
TBR=phajdan.jr for trivial updates of trace_event_analyzer.

Review URL: https://codereview.chromium.org/23799005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229730 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229730